Flow, Turbulence and Combustion最新文献

Planar velocity measurements using the particle image velocimetry technique have been performed at a repetition rate of 10 kHz in the prechamber of a large bore gas engine mounted on a rapid compression machine (RCM), to visualize the velocity fields in the non-reacting gas flow during a compression stroke. The prechamber investigated in this work is a prototype with modifications made to facilitate optical access, and it is mounted axially on the RCM combustion chamber. The parameters of the compression stroke in the RCM are set to achieve a compression ratio of 10. After removing outlying data based on pressure and piston displacement curves, PIV data from compression strokes were analyzed. The time-resolved velocity fields capture the formation and motion of a tumble vortex in the imaged plane. Mean flow fields obtained by phase averaging across the datasets are presented, showing the development of the flow field in the prechamber throughout the compression stroke. The data obtained will be used to validate CFD simulations.

This study investigates laser-induced ignition in a model-rocket combustor through computational simulations. The primary focus is on characterizing successful and unsuccessful ignition scenarios and elucidating the underlying physical mechanisms. Large Eddy simulations (LESs) are utilized to explore laser-based forced ignition in a methane–oxygen combustor, with attention given to the intricate interplay of factors such as initial condition variability and turbulent flow field. Perturbations in laser parameters and initial flow conditions introduce stochastic behavior, revealing critical insights into ignition location relative to the fuel-oxidizer mixture. A significant methodological innovation lies in the adaptation of established image analysis techniques to track and monitor the transport of hot packets within the flow field. By extending these tools, the study provides insights into the interaction between ignition kernels and flammable gases, offering a more comprehensive understanding of the phenomenon. Results highlight the interplay between hydrodynamic ejections from the laser spark and turbulent fluctuations in the background flow. Indeed, the hydrodynamic ejection emanating from the laser spark, which typically plays a central role for isolated kernels in quiescent flows, competes with the entrainment velocity if its values are within the same order of magnitude and if the laser focal location is particularly close to the shear layer’s edge.

A high-order computational fluid dynamics code was developed to simulate the compressible Taylor–Green vortex problem by means of large-eddy (LES) and direct numerical simulations. The code, BASIC, uses explicit central-differencing to compute the spatial derivatives and explicit low storage Runge–Kutta methods for the temporal discretization. Central-differencing schemes were combined with relaxation filtering or with splitting formulas to discretize convective derivative operators. The application of split forms to convective derivatives generally guarantees good stability properties with marginal dissipation. However, these types of schemes were found to be unsuited to perform implicit large-eddy simulations (ILES). The minimally dissipative schemes showed acceptance performance, only when combined with a sub-grid scale model. The relaxation-filtering strategy, on the other hand, although more dissipative, was proven to be more adequate to perform ILES. We showed that reducing the filter dissipation, by optimizing its damping function, has a positive impact in the flow solution. When performing ILES, the utilization of split formulas in conjunction with relaxation filtering has equally yielded promising results. This combined approach enhances numerical stability while preserving low levels of numerical dissipation.

The Benchmarck on the Aerodynamics of a Rectangular 5:1 Cylinder is studied using a data-driven technique which bridges numerical simulation and available experimental results. Because of intrinsic features of the tools used for investigation, in particular in terms of set-up and boundary conditions, significant discrepancies have been observed in the literature when comparing experimental and numerical results. An approach based on the Ensemble Kalman Filter is here used to optimize a synthetic turbulent inlet used as boundary condition in the numerical calculation, in order to reduce the discrepancy with the available experiments. The data-driven method successfully optimizes the boundary condition features, which produce a significant improvement of the accuracy in the prediction of the flow. These findings open perspectives of application towards the analysis of realistic cases, where boundary conditions are complex and usually unknown.

Spatiotemporal wall temperature (T_wall) distributions resulting from flame-wall interactions of lean H₂-air and CH₄-air flames are measured using phosphor thermometry. Such measurements are important to understand transient heat transfer and wall heat flux associated with various flame features. This is particularly true for hydrogen, which can exhibit a range of unique flame features associated with combustion instabilities. Experiments are performed within a two-wall passage, in an optically accessible chamber. The phosphor ScVO₄:Bi³⁺ is used to measure T_wall in a 22 × 22 mm² region with 180 µm/pixel resolution and repetition rate of 1 kHz. Chemiluminescence imaging is combined with phosphor thermometry to correlate the spatiotemporal dynamics of the flame with the heat signatures imposed on the wall. Measurements are performed for lean H₂-air flames with equivalence ratio Φ = 0.56 and compared to CH₄-air flames with Φ = 1. T_wall signatures for H₂-air Φ = 0.56 exhibit alternating high and low-temperature vertical streaks associated with finger-like flame structures, while CH₄-air flames exhibit larger scale wrinkling with identifiable crest/cusp regions that exhibit higher/lower wall temperatures, respectively. The underlying differences in flame morphology and T_wall distributions observed between the CH₄-air and lean H₂-air mixtures are attributed to the differences in their Lewis number (CH₄-air Φ = 1: Le = 0.94; H₂-air Φ = 0.56: Le = 0.39). Findings are presented at two different passage spacings to study the increased wall heat loss with larger surface-area-to-volume ratios. Additional experiments are conducted for H₂-air mixtures with Φ = 0.45, where flame propagation was slower and was more suitable to resolve the wall heat signatures associated with thermodiffusive instabilities. These unstable flame features impose similar wall heat fluxes as flames with 2–3 times greater flame power. In this study, these flame instabilities occur within a small space/time domain, but demonstrate the capability to impose appreciable heat fluxes on surfaces.

In this work, numerical investigations were performed using large eddy simulations and validated against detailed measurements in the CeCOST swirl stabilised burner. Both cold and reactive flow have been studied and the model has shown a good agreement with experiments. The verification of the model was done using the LES index of quality and a single grid estimator. The cold flow simulations predicted results closely to experiments setting baseline for the reactive simulations. Coherent structures like the vortex rope above the swirler and a precessing vortex core in the combustion chamber were identified. The reactive conditions were modelled with the Flamelet generated manifold and artificially thickened flame models. Simulations were performed for an experimental syngas composition from black liquor gasification at three different CO₂ dilution levels. Three different Reynolds numbers were investigated with the model matching closely to experimentally detected 2D flow field and OH for the most CO₂ diluted mixture. It was found that the opening angles of the flames differ by a maximum of 13% between experiments and simulations. The most diluted fuel investigated experienced a liftoff distance of 23.5 mm at the Re 25 k. This was also the highest liftoff distance experienced in this cohort of fuels. The same fuel also proved to have the thickest flame annulus at 78.5 mm. Overall, in cases with no experimental data available the predictions made by the model follow the same trends which hints its applicability to higher Re cases.

A Direct Numerical Simulation (DNS) database of statistically planar flames ranging from the wrinkled flamelets to the thin reaction zones regime and DNS data for a Bunsen premixed flame representing the wrinkled flamelets regime have been utilised to evaluate the fractal dimensions of flame surfaces using the filtering dimension method, the box-counting algorithm and the correlation dimension approach. The fractal dimension evaluated based on the fully resolved three-dimensional data has been found to be reasonably approximated by adding unity to the equivalent fractal dimension evaluated based on two-dimensional projections irrespective of the methodology of extracting fractal dimension. This indicates that the flame surface can be approximated as a self-similar fractal surface for the range of Karlovitz and Damköhler numbers considered here. While all methods, provide results identical to each other for benchmark problems, it has been found that the fractal dimension evaluation based on box-counting method provides almost identical results as that obtained using the filtering dimension method for both three and two dimensions, while the fractal dimensions based on the correlation dimension tend to be slightly smaller. The findings of the current analysis have the potential to be used to reliably estimate the actual fractal dimension in 3D based on experimentally obtained 2D binarised reaction progress variable field. The inner cut-off scales estimated based on all three methodologies yield comparable results in terms of order of magnitude with the box-counting method predicting a smaller value of inner cut-off scale in comparison to other methods. The execution times for fractal dimension extraction based on filtering dimension and box-counting methodologies are found to be comparable but the correlation dimension method is found to be considerably faster than the two alternative approaches and provides results consistent with theoretical bounds in all cases.

The electro-hydro-dynamic-atomization (EHDA) is a well-established technology with numerous micro/nanoparticle fabrication applications. However, a consistent method for explaining the physics behind the process has yet to be established. The present study aims to report a comprehensive non-dimensional analysis to develop a correlation between different process parameters. The dimensionless numbers derived from Buckingham’s pi theorem match well with those derived from the Navier–Stokes equation, establishing the forces involved in EHDA. Flow instability modes during the EHDA process are experimentally visualized using the flow visualization technique and characterized using a microscope. The instability modes are described using derived non-dimension numbers, and results closely align with Ganan-Calvo’s findings. Derived scaling for the current is in good agreement with Ganan-Calvo (1997), which complies with the condition if δ_μ × (Q/Qo)^1/3 >  > 1, then I/Io = 11 × (Q/Qo)^1/4 -5. Moreover, the ratio of ln (Ehd)/ ln (Md) in cone jet mode is found to be ≈2, irrespective of fluids.